1. Field of the Invention
The present invention relates to a pre-reduction furnace for pre-reducing fine iron ore having a wide particle size distribution in a smelting reduction process for producing molten pig iron directly from iron ore and coal, without the need for sintering machines and coke ovens. More particularly, the present invention relates to a 2-stage fluidized bed furnace for pre-reducing fine iron ore which includes a high content of fine particle size ore which is readily decomposed by reaction heat at a pre-reduction temperature and a method for pre-reducing fine iron ore using the pre-reduction furnace.
2. Description of the Prior Art
Generally, methods of reducing iron ore to produce a molten iron include a blast furnace, a direct reduction process by which reduced iron is obtained in a solid form by using a shaft furnace, a rotary kiln and a fluidized bed furnace, as well as other methods using an electric furnace.
The blast furnace is the prevailing iron-making method at the present time. The blast furnace is charged with sintered ore, including fine iron ore, and large amounts of coke, using high quality coking coal as a heat source and as a reducing agent, so as to improve the permeability and reducibility of the iron ore at the shaft portion of the blast furnace.
The blast furnace iron making process has serious drawbacks, such as the immense facility investment, the lack in flexibility of operation due to high tonnage mass production, and the industrial pollution of the environment from the pre-treatment facilities. These problems are caused by the coke ovens required for distilling the coking coal and the facilities needed for manufacturing the sintered iron ore.
A direct reduction process by which iron is obtained by using a high-quality iron ore and a reducing gas from a nature gas has been limitedly employed in geographical areas that can be economically supplied with high quality iron ore and natural gas.
The quantity of crude steel production in the world is relatively low. However, the production of crude steel is gradually increasing, caused by the increasing demand for various kinds of structural steels, as well as by the deterioration in the quality of available scrap.
However, the above method is used through pre-treatments such as pelletizing and sintering to increase permeability when iron ore is charged in a furnace. This is very important in attaining a normal operation. In the case of a smelting reduction process which makes molten iron by directly using fine iron ore of the type having largely varying grain sizes without pre-treatment using coal, the above method has importance as a new method of producing iron capable of replacing a blast furnace. The smelting reduction method has been studied as a means for charging conventional iron ore in a simple reactor and reducing it after directly melting it. Recently, however, a smelting reduction process has been developed for charging iron ore in a melting furnace after pre-reducing a proper quantity in a solid state and finally reducing it. This approach is practical and energy efficient.
It is of interest to develop a technique employing a fluidized bed pre-reduction furnace which fluidizes fine iron ore by using an off-gas from a smelting reduction furnace supplied to the fluidized bed reduction furnace, which is known as a method of processing fine iron ores for pre-reduction of this process.
In a conventional fluidized bed direct reduction process related to such techniques, it is practical to use special fine particle size iron ore in which the particle size distribution is narrow. A fluidized bed reduction process using fine iron ores having a wide distribution of particle size (i.e., pre-reduction step of a smelting reduction process producing molten iron using coal as a heat source and as a reducing agent and iron ores without pre-treatment) has recently been developed but is not yet practical because of technical difficulties encountered. In a known smelting reduction process producing molten iron which directly uses fine iron ore and coal without pre-treatment, the smelting reduction comprises a smelting reduction furnace and a pre-reduction furnace as depicted in FIG. 1. The smelting reducing furnace generates pre-reduction gas by a gasification reaction of the charged coal and then supplies the pre-reduction gas to the pre-reduction furnace through a gas duct after collection of dust in a hot cyclone. The pre-reduction gas pre-reduces the iron ore charged in the pre-reduction furnace to a high degree by using a reducing force and heat provided by an off-gas of the smelting reduction furnace. Pre-reduced iron ore, which is charged in the smelting reduction furnace, is converted into molten metal by finally being reduced in the molten state.
A method of reducing fine iron ores which have a wide particle size distribution by a fluidized bed pre-reduction process is shown in FIG. 2 involving a circulating fluidized bed pre-reduction process in which coarse particle size iron ore is reduced by fluidization in the lower zone thereof, and small particle size iron ore is discharged by elutriating through a freeboard area of the top of the furnace. The ore fines are then collected in a hot cyclone and a portion of the reducing gas supplied to the hot cyclone is fed into a fluidized bed formed in the lower zone through a downcomer. However, it is not easy to control the state of fluidization of charging materials having different fractions of grain sizes.
To operate the fluidized bed with fractions of different grain sizes, it is necessary to form a base of coarse particle size iron ore in the lower bed layer. The fluidization velocity causes discharge by segregation of medium and small particle size ore with coarse particle size ore, and it raises a problem of ore collection due to a deterioration in the collection efficiency of the cyclone fixed at the rear stage.
In FIG. 2, reference numbers 1, 2 and 3 indicate, respectively, a pre-reduction furnace, a hot cyclone, and a gas distributing plate fixed in the pre-reduction furnace.
As described above, to solve the problem in a conventional fluidized reduction furnace using fine iron ore which has a wide particle size distribution, a circulating fluidized bed pre-reduction furnace for pre-reducing fine iron ores composed of 2-stage fluidized bed furnaces is disclosed in Korean patent publication Nos. 94-1137 and 94-1138. In the process of the fluidized bed furnace, if an iron ore burden having a wide particle size distribution is charged in a first fluidized bed reduction furnace, medium and small particle size ore is separately supplied to a second fluidized bed furnace by a fluidizing gas. The coarse particle size ore is stably reduced by fluidizing in the first fluidized bed pre-reduction furnace, and the medium and small particle size ore is reduced in the second fluidized bed pre-reduction furnace, under a different particle size distribution.
Fine iron ore having a wide particle size distribution is reduced in a fluidized state by continuously charging the fine ore in the first fluidized bed reduction furnace. In the process using a 2-stage fluidized bed furnace, a gas fluidizing velocity for the fluidized bed should be operated on a basis of a large particle size ore.
Accordingly, in the case of the circulating fluidized bed pre-reduction furnace for pre-reducing fine particle size iron ore, a large quantity of coarse particle size iron ore can be transferred to the top of the first fluidized bed reduction furnace by elutriating with the small particle size ore. There is, however, a problem that deteriorates fluidization of the fine particle size iron ore in the second fluidized bed reduction furnace due to presence of the coarse particle size iron ore.
Furthermore, in the case of a fluidized bed reduction furnace having two reactors, the reducing gas supplied to the first fluidized bed reduction furnace has a problem that deteriorates the reducing force of the fluidizing gas in the second fluidized bed stage. The gas being supplied to the second fluidized bed reduction furnace is oxidizing to a high degree, because the coarse particle size ore is reduced by fluidizing while the small particle size ore is supplied separately to the second fluidized bed reduction furnace.
The present inventor has studied these various problems arising in the conventional method described above and, as a result, has conceived the present invention.